Ultra high density, non-volatile ferromagnetic random access memory

ABSTRACT

A random access memory element utilizes giant magnetoresistance. The element includes at least one pair of ferromagnetic layers sandwiching a nonmagnetic conductive layer. At least one of the two ferromagnetic layers has a magnetic moment oriented within its own plane. The magnetic moment of at least the first ferromagnetic layer of the pair has its magnetic moment oriented within its own plane and is typically fixed in direction during use. The second ferromagnetic layer of the pair has a magnetic moment which has at least two preferred directions of orientation. These preferred directions of orientation may or may not reside within the plane of the second ferromagnetic layer. The bit of the memory element may be set by applying to the element a magnetic field which orients the magnetic moment of the second ferromagnetic layer in one or the other of these preferred orientations. Once the bit is set, the value of the determined by the relative alignment of the magnetic moments of the first and second ferromagnetic layers. This value may be read by applying an interrogating current across the memory element, perpendicular to the plane within which the magnetic moment of the first ferromagnetic layer is oriented, and observing the variation in resistance. These ferromagnetic elements may be fabricated using conventional photolithography. Groups of these ferromagnetic element may be organized into word trees and other arrays.

This is a division of copending application Ser. No. 08/130,479, filedOct. 1, 1993, to Gary A. Prinz, titled ULTRA HIGH DENSITY, NON-VOLATILEFERROMAGNETIC RANDOM ACCESS MEMORY.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to ferromagnetic memory and morespecifically to ferromagnetic memory utilizing giant magnetoresistanceand spin polarization.

2. Description of the Background Art

For many years, random access memory for computers was constructed frommagnetic elements. This memory had the advantage of very highreliability, nonvolatility in the event of power loss and infinitelifetime under use. Since this memory was hand assembled fromthree-dimensional ferrite elements, it was eventually supplanted byplanar arrays of semiconductor elements. Planar arrays of semiconductorscan be fabricated by lithography at a much lower cost than the cost offabricating prior art magnetic ferrite memory elements. Additionally,these semiconductor arrays are more compact and faster than prior artferrite magnetic memory elements. Future benefits of increasinglysmaller scale in semiconductor memory are now jeopardized by the concernof loss of reliability, since very small scale semiconductor elementsare not electrically robust.

Non-volatile magnetic memory elements that are read by measuringresistance have been previously demonstrated by Honeywell Corporation.These systems operate on the basis of the classical anisotropicmagneto-resistance phenomena, which results in resistance differenceswhen the magnetization is oriented perpendicular versus parallel to thecurrent. Previous work by others has shown that a 2% change inresistance is sufficient to permit the fabrication of memory arrayscompatible with existing CMOS computer electronics. Unfortunately,scaling of these elements down from the current 1 μm size has provedchallenging.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to produce an inexpensivenon-volatile random access ferromagnetic memory.

It is another object of the present invention to produce a non-volatileferromagnetic random access memory that is faster than the presentlyavailable semiconductor random access memory.

It is a further object of the present invention to produce a highlycompact non-volatile random access ferromagnetic memory.

These and additional objects of the invention are accomplished by anon-volatile random access memory element that employs giantmagnetoresistance (GMR), i.e., the spin-valve effect. The memory elementhas a sandwich structure in which layers of ferromagnetic material, atleast one of which has its magnetic moment oriented within the plane ofthe layer, are spaced apart by a layer of a non-magnetic metal.Conducting leads provide current to pass through the ferromagneticlayers, perpendicular to the magnetic moment of the at least oneferromagnetic layer having its magnetic moment oriented within the planeof the ferromagnetic layer. Between and in physical contact with one ofthe ferromagnetic layers and the conducting leads there may be anantiferromagnetic layer. The antiferromagnetic layer fixes the directionand magnitude of the magnetic moment of the ferromagnetic layer that itcontacts.

When a voltage is applied across the two ferromagnetic layers, theresistance varies depending upon whether the magnetic moments of theselayers are aligned in the same direction with respect to each other.Resistance between the two layers increases when the magnetic moments ofthese two ferromagnetic layers are not aligned in the same direction,i.e, misaligned or anti-parallel (anti-aligned). The resistance betweenthe two layers drops when the magnetic moments of these twoferromagnetic layers are in essentially the same direction (parallel) ormove from a more anti-parallel orientation to one which is moreparallel. The more parallel state can be assigned a value of "0" or "1"while the more antiparallel state can be assigned, respectively, a valueof "1" or "0". Thus, the alignment status of each memory elementaccording to the present invention represents a bit of information.

The bit can be altered in a memory element according to the presentinvention by applying an sufficiently high current in the conductingleads in order to generate a magnetic field sufficient to align, in onedirection, the magnetic moment of any unpinned ferromagnetic layer alongone of the easy directions of orientation. The direction of orientationfavored by the orienting current is of course determined by the polarityof that orienting current. Once set, the bit may be read by applying asmaller current through the appropriate conducting leads and determiningwhether the resistance is more or less than that of a referenceresistance.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Preferred Embodimentsand the accompanying drawings in which like numerals in differentfigures represent the same structures or elements, wherein:

FIG. 1 shows a non-volatile random access memory element according to apreferred embodiment of the present invention.

FIG. 2 is a top view of an array of non-volatile random access memoryelements according to the present invention.

FIG. 3 shows a partial misalignment of the magnetic moments of twoferromagnetic layers in a non-volatile random access memory elementaccording to FIG. 1.

FIG. 4 shows a second embodiment of a non-volatile random access memoryelement according to a preferred embodiment of the present invention.This embodiment employs alternating hard and soft ferromagnetic layers.

FIGS. 5a and 5b schematically shows the two "at rest" configurations ofa non-volatile random access memory element according to the presentinvention which employs alternating hard and soft ferromagnetic layers.

FIG. 6 is a top view of an array of non-volatile random access memoryelements according to the present invention, illustrating the means foraccessing and poling the memory elements in the array.

FIG. 7 is a top view of a 5-bit word tree including random access memoryelements according to the present invention.

FIG. 8 shows a first step in an exemplary process for manufacturing anarray of random access memory elements according to the presentinvention.

FIG. 9 shows a second step in an exemplary process for manufacturing anarray of random access memory elements according to the presentinvention.

FIG. 10 shows a third step in an exemplary process for manufacturing anarray of random access memory elements according to the presentinvention.

FIG. 11 shows a fourth step in an exemplary process for manufacturing anarray of random access memory elements according to the presentinvention.

FIG. 12 shows a fifth step in an exemplary process for manufacturing anarray of random access memory elements according to the presentinvention.

FIG. 13 shows a sixth step in an exemplary process for manufacturing anarray of random access memory elements according to the presentinvention.

FIGS. 14a and 14b show a planar striped non-volatile random accessmemory element according to the present invention, in two different "atrest" configurations.

FIG. 15 shows an array of planar striped non-volatile random accessmemory elements according to FIG. 14.

FIGS. 16a, 16b, 16c and 16d are a series of Figures illustrating thefabrication and a non-volatile random access memory element according tothe present invention.

FIG. 17. is a planar view of another embodiment of a non-volatileferromagnetic memory element according to the present invention.

FIG. 18 is a planar view of yet another embodiment of a non-volatileferromagnetic memory element according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The carriers in devices can be identified not only as electrons andholes, but also by their spin state being "up" or "down". Just aspolarized light may be easily controlled by passing it through crossedpolarizers, spin polarized electron current can be created, controlledand measured by passing it between magnetic films whose relativemagnetic moments can be rotated. The spin polarization manifests itselfas an extra resistance in a magnetic circuit element, commonly referredto as magneto-resistance. The modern manifestation of magneto-resistanceshould not be confused with older observations common to semiconductorsand metal in which the carriers are merely deflected by the classicalLorentz force (V×B) in the presence of a magnetic field. This moderneffect is purely quantum mechanical and occurs when two ferromagneticmetals are separated by a non-magnetic conductor. When a bias voltagecauses carriers to flow from one magnetic metal into the other throughthe intervening conductor, the spin-polarization of the carriers canplay a dominant role. The carriers leaving the first ferromagnetic metalare highly polarized because they are emitted from band states which arehighly polarized. The resistance which they meet in trying to enter thesecond ferromagnetic layer depends strongly upon the spin polarizationof the states available to them. If the ferromagnetic moments of the twomagnetic metals are aligned, then the spin descriptions of the statesare the same in the two materials and carriers will pass freely betweenthem. If the two moments are anti-aligned, then the states areoppositely labeled (i.e., "up" in the first ferromagnetic layer is"down" in the second ferromagnetic layer), the carriers will find thatthey have fewer states to enter and will experience a higher resistance.This phenomenon is now commonly referred to as the spin-valve effect. Bysimply measuring the resistance between two magnetic layers, one candetermine if their magnetic moments are parallel or anti-parallel.

The present invention uses this spin-valve effect as the basis of amemory element. In the memory element of the present invention, the twostates, parallel and anti-parallel, represent two bits, "0" and "1". Ofcourse, each state may be arbitrarily assigned a value of "0" or "1",provided that when the parallel state is assigned a value of "0", theantiparallel state is assigned a value of "1" and when the parallelstate is assigned a value of "1", the antiparallel state is assigned avalue of "0". The state of the memory element may be readilyinterrogated by measuring the resistance.

Most early research into the spin-valve effect measured in-planetransport and relied upon electrons being scattered between magneticlayers as the electrons propagated parallel to the magnetic layers. Thisnon-optimum orientation has yielded changes in resistance of ΔR/R=0.45in multilayered sandwiches at room temperatures. The latest work, andthe present invention, propagate the current perpendicular to the layers(i.e., perpendicular spin-polarized transport), which maximizes theeffect and yields up to an order of magnitude increase in ΔR/R. Thismaximization results from the elimination or reduction of the shuntingeffects of the non-magnetic layers in parallel transport. In paralleltransport systems, as opposed to the perpendicular transport systems ofthe present invention, these shunting effects permit a considerablenumber of non-spin scattered evens to occur, thus diluting themagneto-resistance effect.

To obtain a useful magneto-resistance effect in spin-polarized systems,the polarized carrier should travel through the memory element over atime shorter than its relaxation time. The mechanisms which reverse thespin of a polarized carrier (e.g., spin-orbit scattering from defects orimpurities; domain walls; interface roughness; non-uniformmagnetization; and crystal structure changes), resulting in thescattering, are complex and the interactions of these mechanisms are notentirely understood. Nevertheless, it is now clear that most of the ΔR/Reffects come from interfacial spin scattering rather than bulkscattering within the interior of the ferromagnetic layers. Therefore,the ferromagnetic and other layers of any memory element made accordingto the invention should be as thin as possible. Indeed only a thicknessof only a couple of atomic layers in the ferromagnetic films layers areneeded to produce the required spin polarization. Additionally, both themagnetic and non-magnetic layers should be as small as possible, whichimplies high purity materials in defect-free structures. Thissignificance of defect-free structures is supported by results showingthe well-matched crystal structures (Fe/Cr and Cu/Co) give largerspin-polarization effects than do unmatched systems such as Co/Ag.

The present application specifically describes two different preferredapproaches for the geometry of magnetic memory elements andtwo-dimensional arrays which incorporate them. Although both of thesesapproaches depend upon the spin-valve effect, the first approach uses asandwich structure, similar to quantum dots in semiconductor technology,while the second approach uses a planar sequence of magnetic metalstripes. The former structure is most easily fabricated using highresolution lithography, while the latter structure is most easily formedusing focused ion beam milling in situ.

In one approach, the magnetic memory element includes at least severalvertically stacked layers. One simple example of this vertically stackedstructure is shown in FIG. 1 and may be used to illustrate some of thebasic concepts of the present invention. Memory element 10 in FIG. 1includes a bottom conducting lead 12. The upper surface of bottomconducting lead 12 supports and physically contacts the lower surface oflayer 11 of non-magnetic, metallic material, for example, Cu. The uppersurface of non-magnetic, conducting layer 11 supports and physicallycontacts layer 14 of antiferromagnetic metal (for example, FeMn).Throughout the specification and the claims that follow, it should beunderstood the terms "upper" and "lower" are used as terms convenienceto distinguish various surfaces relative to each other. Neither "upper"nor "lower", as used in the specification and claims that follow, implythe orientation of any element with respect to the gravitational field.)

The lower surface of layer 16 of ferromagnetic metal, for example, Co,rests on and physically contacts the upper surface of the non-magnetic,conducting layer 11. Ferromagnetic layer 16 is deposited, using knownand conventional poling means, so that the magnetic moment within thatlayer will have its preferred direction of orientation within the planeof the layer (i.e., not extending out of the layer toward adjacentlayers of stack 10). Antiferromagnetic layer 14 functions to "pin" theorientation of the magnetic moment within ferromagnetic layer 16 in thispreferred orientation. This pinning essentially prevents any changes inthe orientation of the magnetic moment of ferromagnetic layer 16 duringnormal use.

The lower surface of non-magnetic layer 18 rests on and physicallycontacts the upper surface of ferromagnetic layer 16 and serves as theintervening, non-magnetic layer required for the spin-valve effectdiscussed above. The upper surface of non-magnetic layer 18 supports andphysically contacts the ferromagnetic layer 20. Ferromagnetic layer 20is deposited, using known and conventional means, including poling, sothat the magnetic moment within that layer will have two preferreddirections of orientation laying within the plane of the layer (i.e.,not extending out of the layer toward adjacent layers of stack 10),e.g., parallel and anti-parallel with respect to the magnetic moment offerromagnetic layer 16. The bottom surface of top conducting lead 22rests upon and physically contacts the upper surface of ferromagneticlayer 20.

In use, applying a sufficiently large current pulse (an interrogationpulse) down the length of bottom conducting lead 12 temporarilydisplaces the orientation of the magnetic moment within ferromagneticlayer 20 away from its preferred orientation and either closer to orfurther from parallel alignment with the orientation of the magneticmoment of ferromagnetic layer 16, depending upon the originalorientations of the magnetic moments of ferromagnetic layers 16 and 20with respect to each other.

To permit reading of memory element 10, bottom conducting lead 12 iselectrically biased with respect to top conducting lead 10 within aconstant current circuit (a circuit which remains at constant currentunless disturbed by an externally applied pulse) (not shown). Theresistance between bottom conducting lead 12 and top conducting lead 10is monitored. During this monitoring, a small current pulse (theinterrogating pulse) is applied between bottom conducting lead 12 andtop conducting lead 10. This interrogating pulse temporarily displacesthe magnetic moment of ferromagnetic layer 20 away from its most stableorientation and alters the resistance of the stack. This bump inresistance is detected by the resistance monitoring circuitry (notshown) which measures the derivative of the slope of the resistance inthe circuit. Which state (increase in resistance vs. decrease inresistance) is assigned the value of "0" and which state is assigned thevalue of "0" is defined in and by the electronic circuit of the device(not shown) employing memory element 10.

Although the largest magnetoresistance effects would be observed forcases where the magnetic orientations of the ferromagnetic layers 16 and18 may be either parallel (aligned) or antiparallel (antialigned) withrespect to each other, for the purposes of an actually operatingelement, it may often be better to sacrifice some of the change inmagnetoresistance for ease of fabrication and operation. For example,FIG. 3 shows the orientation of the ferromagnetic layers memory elements50 and 52. The intervening non-ferromagnetic layers 53 are indicated bythe dashed lines. As indicated by the solid arrows, the bottomferromagnetic layers 54 of memory elements 50 and 52 both have theirmagnetic moments pinned in the same direction.

The top ferromagnetic layers 56 and 58 of elements 50 and 52 are bothfabricated to have two preferred orientation directions for the magneticmoment. In the embodiment of FIG. 3, these orientation directions areperpendicular with respect to each other. One of these preferredorientation directions, shown by the solid arrow in layer 56, isarbitrarily assigned a value of "0". The other direction, shown by thesolid arrow in layer 58, is arbitrarily assigned a value of "1". Theresistance of these two orientations is the same, since the anglebetween the two moments is the same. However, if an interrogatingcurrent pulse is propagated down the length of the bottom conductor (seeFIG. 1) of each of memory elements 50 and 52, it will generate amagnetic field perpendicular to the conductor, which will act on layers56 and 58. This magnetic field will rotate the orientations of layers 56and 58 in the direction indicated by the curved dotted arrows. (For thepurposes of illustration, this rotation has been illustrated asclockwise, although the rotation can be made counter-clockwise byreversing the polarity of the current applied across the bottom or topconductor.

The effect of this rotation upon the magnetoresistance in memoryelements 50 and 52, however, will be opposite. The magnetic moment oflayer 56 rotates to the orientation indicated by the dashed arrow, whichis closer to an antialigned state, thus increasing the magnetoresistanceacross element 50. On the other hand, this same clockwise rotationrotates the magnetic moment of layer 58 to the orientation indicated bythe dashed arrow, which is closer to an aligned state, thus decreasingthe resistance across element 50. These changes are readily measured byrunning a read current through the stack, which reveals the bit to be a"0" or "1".

In order to set (i.e., write) a bit in memory element 50 or 52,simultaneous current pulses are sent through the upper and lower currentleads that intersect the selected element. Depending upon the polarityof the current, the resulting fields will either leave the bit as a "0"or a "1". FIG. 2 shows an array 59 of memory elements 50 and 52 in apreferred arrangement that minimizes the fringing fields from theelement in order to prevent cross-talk between neighboring elements. Topconducting leads 60 and bottom conducting leads 62 define a gridpattern. Memory elements 50 and 52 are sandwiched at the points wheretop conducting leads 60 cross bottom conducting leads 62. Otherarrangements of memory elements 50 and 52 within array 59 are possible.

Because ΔR/R effects come primarily from interfacial spin scattering, itis desirable to provide memory elements having a multilayer structure,with a maximum of interfaces being forced into a length less than thespin-relaxation length. To this end, a multilayered memory elementaccording to the present invention includes alternating layers of hardmagnetic and soft magnetic materials spaced from each other byintervening layers of nonmagnetic material. In these multilayeredstructures, an antiferromagnetic pinning layer may be used, but is notnecessary.

FIG. 4 shows a memory element 100 having a plurality of interfaces whichinteract with spin-polarized carriers. The structure of memory element100 is similar to the structure of memory element 10 shown in FIG. 1.Except where noted, analogous structures perform analogous function, andessentially the same considerations apply when selecting appropriatematerials and dimensions. Memory element 100 includes bottom conductinglead 102. The upper surface of bottom conducting lead 102 supports andphysically contacts the lower surface of layer 104 of non-magneticmaterial. The upper surface of optional antiferromagnetic layer 101, ifpresent, is sandwiched between and physically contacts layer 104 ofnon-magnetic material and layer 106 of hard ferromagnetic metal. Ifoptional antiferromagnetic layer 101 is absent, the lower surface oflayer 106 of hard magnetic ferromagnetic metal, for example, Co, restson and physically contacts the upper surface of the non-magnetic layer104. Hard ferromagnetic layer 106 is deposited, using known andconventional means, so that the magnetic moment within that layer willhave its preferred direction of orientation within the plane of thelayer (i.e., not extending out of the layer toward adjacent layers ofmemory element 10).

The lower surface of non-magnetic layer 108 rests on and physicallycontacts the upper surface of hard ferromagnetic layer 106 and serves asthe intervening, non-magnetic layer required for the spin-valve effect.The upper surface of non-magnetic layer 106 supports and physicallycontacts the soft ferromagnetic layer 110. Ferromagnetic layer 110 isdeposited, using known and conventional means, so that the magneticmoments within that layer will have two preferred directions oforientation. These directions lie within the plane of the layer (i.e.,they do not extend out of the layer toward adjacent layers of memoryelement 100). The bottom surface of layer 111 of conductive,non-magnetic material 112 rests upon and physically contacts the uppersurface of ferromagnetic layer 110. The bottom surface of top conductinglead 112 also rests upon and physically contacts the upper surface offerromagnetic layer 110 and physically contacts the upper surface oflayer 111 of conducting, non-magnetic material.

Memory element 100 uses alternating ferromagnetic layers of two types,hard (type H, layer 106) and soft (type S, layer 110). The type H layerhas a high coercive field (at least about, and preferably greater than,100 Oe), H_(H), i.e., magnetically hard, while the type S layer has alow coercive field (less than 100 Oe), type H_(S), i.e., magneticallysoft. Using this arrangement, one can readily switch the sandwich froman aligned to anti-aligned state by merely reversing the magnetizationof the soft ferromagnetic layer 110, while leaving the magnetizationdirection of the hard ferromagnetic layer 106 fixed.

This alternating arrangement is better illustrated in the multilayerstack 200 of FIGS. 5a and 5b. Although stack 200 has more layers, it isotherwise analogous in form and function to the stack of memory element100 in FIG. 4. Neither the top nor bottom conducting leads, nor theoptional bottom, antiferromagnetic layer are shown in FIGS. 5a and 5b.The ferromagnetic layers 206, 210, 214 and 218 alternate between hard(labeled "H") and soft (labeled "S") materials. Sandwiched between eachpair of hard magnetic and soft magnetic layers (204/206; 206/208;208/210) is a layer of nonmagnetic material 212, 214, 216, respectively.

Operationally, the memory element 200 has two "at rest" configurationswhich arbitrarily define a "0" (FIG. 5a) and "1" bit (FIG. 5b). In theembodiment of FIGS. 5a and 8b, both of these configurations areanti-aligned ("anti-ferromagnetic") sequences, but with reversed phase.That is, the "0" bit has the hard layers all pointing to the left, whilein the "1" bit they all point to the right. In both cases the softlayers are anti-aligned with the hard layers. In order to interrogatethe stack, one merely applies a pulse field H>H_(S) sufficient toreverse the soft layers that are oriented antiparallel to the appliedpulse field. The applied pulse, is of insufficient magnitude to reversethe hard layers. This reversal will be accompanied by a resistancechange ΔR/R if the applied field H is antiparallel to the soft layers,but no change if it is parallel to them. The pulsed "read" field isprovided by an overlay current drive line (not shown). Fields of 10 Oeto 100 Oe, necessary for reading or switching, are readily obtained withpulses of amp peaks in existing technology.

In order to "write" a bit, the pulsed field is increased such thatH>H_(H), and the hard layers will reverse. The multilayer system will ingeneral always revert to the anti-aligned state since it is the minimumenergy state to provide flux closure of all the magnetic layers. Ifnecessary, a lower level pulse H_(H) >H>H_(s) can be provided to restoreall of the soft layers without altering the hard ones.

The two anti-aligned closed magnetic circuit "at-rest" configurationsshown in FIGS. 5a and 5b are extremely important in order to eliminatefringing fields from the elements. During interrogation, however, thereis a momentary alignment which will generate a pulsed fringing field.The magnitude of this field will limit the closest approach ofneighboring elements, hence the ultimate packing density. The moststraightforward engineering solution to this problem is to provide asoft magnetic "keeper" shunt at the top of the stack above the driveline to collect the fringing field flux lines and prevent theirextending out to neighboring elements. On a more sophisticated level,the stray fringing field can be reduced by lowering the magnetic momentsof the layers. Thus, the soft and hard layers themselves may befabricated as trilayer structures containing ultrathin layers ofmoment-carrying magnetic material. For example, others have demonstratedthat FeMn, an anti-ferromagnet with no net moment, can magnetically pinan adjacent magnetic layer via exchange coupling. Since all of the spintransport effects essentially are determined by the polarization of theinterfaces, the "hard" layer or layers could be composed of the sandwich(Co/FeMn/Co) or (Fe/FeMn/Fe). The polarization of the highly polarizedinterface layers of Co or Fe would remain fixed in direction belowfields of 200 Oe. Similarly the soft layer could be formed of(Fe/FeNi/Fe) or (Co/FeNi/Co). The FeNi (permalloy) layer is very soft,easily switched, but has a small moment and would contribute littlefringing field.

FIG. 6 shows a simplified matrix 300 consisting of two independentarrays 302, 304 of parallel current carrying bars 306 and 308.Preferably, arrays 302 and 304 are oriented about 90° to each other.Current carrying bars 306 of array 302 are above and not directly incontact with current carrying bars 308 of array 304. Current carryingbars 306, 308 from arrays 302 and 304 are connected at theirintersections by the stacked memory elements 310 sandwiched betweenthem. The interrogation current to any given element comes in one end ofthese bars, 306 or 308, passes through the element and passes outthrough an end of the other respective contacting bar 308 or 306. Theother ends of bars 306 and 308 provide the leads for measurement of thepotential drop, thus providing a true 4-point probe measurement whicheliminates the lead resistance in the circuit. For purposes ofillustration only, FIG. 6 shows interrogation current 312 (shown by thelines labelled "J", with the arrow pointing in the direction of currentflow) entering through end 314, flowing through a stack 310 and exitingthrough end 316 of a current carrying bar 308 of array 304. Resistanceis measured at ends 320, 322 of the specified current carrying barsemployed in this example that oppose ends 314 and 316, respectively.

In order for the x-y matrix of FIG. 6 to give true measurements of asingle element, parallel paths of conduction must be eliminated. Thisgoal is accomplished by providing diode film elements 322 at one end ofthe stack where contact to the bars are made, either above or below thecurrent bar through which the current enters the stack. This arrangementwill prevent any competing currents since all currents are now forced topass one way through the elements.

FIG. 7 shows a 5-bit word tree 400 consisting of one underlying base bar402 crossed by five overlayer bars 404 with a stacked memory element 406at each intersection.

FIGS. 8 through 11 illustrate one exemplary method for making a memoryelement according to the present invention. As shown in FIG. 8, wafer500 includes a top conductive layer 501 and magnetic multilayerstructure 502 is first deposited on a thick base conducting layer 504 onan insulating substrate 506. In the embodiment of FIG. 8, insulatingsubstrate 506 is silicon 508 on silicon oxide 510. Of course, thesubstrate employed is not critical to the present invention. Otherinsulating substrates used in the fabrication of electronic memoryelements may be used.

Next, as shown in FIG. 9, a photoresist layer 512 is deposited over topconductive layer 501 and lines 514 (typically on the order of about 1 μmthickness) are defined in the resist by conventional photolithographicmethods. Then, as shown in FIG. 10, the portions of multilayer 506exposed by lines 514 are ion milled down to insulating substrate 506 andphotoresist layer 512 is removed.

Subsequently, as shown in FIG. 11, the upper surface of assembly shownin FIG. 10 is planarized with a layer 516 of insulator, such aspolyamide, SiO or SiNi. This surface is then planarized down to thesurface of 514 by a suitable etching or milling technique. Layer 519 ofa different photoresist material is then deposited over the planarizedsurface formed by the upper surfaces of metal lines 514 and insulatinglayer 516. Lines 518 (typically on the order of about 1 micron wide) arethen defined in photoresist layer 519, perpendicular to metal lines 514(FIG. 12). A conductive metal is then deposited, filling lines 518 withconductive metal. After photoresist layer 518 is removed, conductivemetal lines 520 remain and contact the top surfaces of metal lines 514.

FIG. 14 a and FIG. 14b show a planar memory element 700 according to thepresent invention. In this embodiment, the conductive path lies entirelywithin the plane of element 700. Conducting layers 702 and 704 sandwicha multilayer structure in which ferromagnetic layers 706 and 708sandwich a non-ferromagnetic layer 710. As with the other embodiments ofthe present invention, the ferromagnetic layer 706 is pinned (byexchange biasing, typically using an antiferromagnetic layer sandwichedbetween and in contact with layers 706 and 702 (not shown)) and theferromagnetic layer 708 is fabricated to have two preferred directionsof orientation (FIG. 14a vs. FIG. 14b). In this embodiment, onlyferromagnetic layer 706 has its magnetic moment oriented perpendicularto the conductive path. The operation of the ferromagnetic memoryelements of FIGS. 14a and 14b are analogous to that described for theembodiment of FIG. 1. Additionally, the planar memory element inaccordance with the present invention can be adapted to use the allmaterials and to employ all modifications useful in the design ofvertically stacked elements (for example, the use of alternating "hard"and "soft" ferromagnetic layers).

FIG. 15 shows a random access x-y array 800 of planar ferromagneticelements 700 according to the present invention. Service line 802contact memory elements 700 at the upper surface of conducting layer704, while service line 804 contacts the lower surface of conductinglayer 702. The diagonal array of elements minimizes the overlap ofservice lines 802 and 804.

Service lines 802 and 804 and conductors 702 and 704 may be readilymanufactured by routine making and deposition processes. The multilayerstructure including layers 706, 708 closely separated by layer 710 mustbe fabricated by a technique which will provide contamination freeinterfaces, for example, in-vacuum processing.

FIGS. 16a, 16b, 16c and 16d show an in-vacuum line process useful formaking the arrays of planar magnetic memory elements shown in FIG. 15.After preparing the lattice of service lines (only line 802 is shown inFIGS. 16a-16d) upon an insulating substrate 804, one is left withconducting pads 806 (typically Cu) upon which the striped memory elementwill be constructed. Wafer 900 is totally covered with photoresist layer902 and placed in a vacuum chamber (not shown). The locations, trenches904 and 908, of ferromagnetic strips 704 and 706, respectively, areselectively milled away down to top surface substrate 804 or below witha focused ion beam (FIG. 16b). As shown in FIG. 16c, ferromagnetic metal(e.g., Co) layer 904 is carefully deposited to completely fill trenches906 and 908 in order to make good electrical contact to the exposed Cuwalls 702a and 704a. Wafer 900 is removed from the vacuum chamber andphotoresist layer 902 is removed, thus removing the excess ferromagneticmetal from layer 904 and leaving the striped memory element 700.

A ferromagnetic random access memory element according to the presentinvention may also be realized utilizing an annular configuration, asshown in FIG. 17. In one annular configuration 1000, the outermost ring1002 is a conductive, nonmagnetic metal. A conductive layer or line 1004(e.g., solid rod, hollow tube, annular ring, or solid dot) is positionedat the center of the annulus. Pairs of ferromagnetic layers 1006, 1008,each pair sandwiching a conductive, nonmagnetic metal layer 1010, residebetween the outermost layer 1002 and innermost layer or line 1004. Eachferromagnetic layer 1006, 1008 is poled so that the easy axis of itsmagnetic moment μ is oriented to be either clockwise orcounterclockwise. In order to pin the magnetic moment of the outermostferromagnetic layer, an antiferromagnetic layer 1112 may be positionedbetween the outermost conductive layer and the outermost ferromagneticlayer. In this arrangement, the resistance of the element to the radialcurrent flow J the would depend upon whether the magnetic moments μ ofthe ferromagnetic layers 1006, 1008 are all oriented in the samedirection (either clockwise or counterclockwise), i.e., aligned, orwhether the magnetic moments μ of the ferromagnetic layers 1006, 1008alternate sequentially between clockwise and counterclockwise andcounterclockwise orientations, i.e, anti-aligned, as shown.

In another annular configuration (FIG. 18), the memory element iscomposed of a stack 2000 of rings 2002 (nonmagnetic metal), 2004(ferromagnetic, magnetic moment μ as shown by arrow), 2006 (nonmagneticmetal), 2008 (ferromagnetic, magnetic moment μ as shown by arrow), 2011(nonmagnetic metal), the ferromagnetic layers 2004 and 2008 having thesame (as shown) or alternating directions of magnetization, analogous toFIG. 4, with electrical contacts 2010, 2012 at the top and bottom ofstack 2000 for interrogating the magnetoresistance of the stack. Thecenter of the stack, however, has been replaced by a conducting "rod"2013, which is insulated from the stack elements themselves byinsulating layer 2014. If needed or desired, antiferromagnetic layer2016 is sandwiched between nonmagnetic layer 2002 and ferromagneticlayer 2004. Current flow J is axial through the stack elements.

In both of the annular configurations described above, the magneticelements are constructed to be closed magnetic circuits, either hollowcylinders in the first configuration, or hollow washers in the secondconfiguration. These closed magnetic circuits have no fringing fieldsand therefore permit adjacent memory elements to be packed very densely.In both cases, in order to "write" information in an element, themagnetic flux must be generated in a circular pattern either parallel oranti-parallel to the flux in the annular magnetic components. One way todo this is via the center electrical conductor, 1004 or 2013. Of course,other techniques may be realized to achieve this operation.

The ferromagnetic metal layers according to the present invention may bemade of any ferromagnetic material. For example, the ferromagnetic layermay be Fe, Co, Ni or an alloy thereof (including permalloy and themagnetic alloys described in U.S. Pat. Nos. 4,402,770 and 4,402,043,both to Norman C. Koon, and both incorporated herein in theirentireties). The non-ferromagnetic layer may be any electricallyconductive, non-ferromagnetic metal, e.g., Cu, Pt, Ag, Au and alloysthereof. If hard and soft ferromagnetic layers are used, the hardmagnetic material may be, for example, Co, Fe or an alloy thereof(including the hard, supermagnetic alloys described in U.S. Pat. No.4,402,770 to Norman C. Koon) while the soft magnetic material may be,for example, Ni, Fe, Co and alloys thereof (including permalloy and thesoft magnetic materials described in U.S. Pat. No. 4,409,043 to NormanC. Koon). The anti-ferromagnetic layer, if used, may be, for example,Cr, Mn or an alloy thereof, e.g., FeMn or a rare-earth-containingmaterial.

Typically, the ferromagnetic layers used in the present invention have athickness of from about 10 Å to about 100 Å, preferably about 50 Å toabout 100 Å for ease of fabrication. Similarly, the non-ferromagneticlayers used in the present invention have a typical thickness of from 10Å to about 100 Å, preferably about 50 Å to about 100 Å for ease offabrication.

In the fabrication techniques described above, the photoresist materialand insulating layers used are not particularly critical. Anyphotoresist materials and any insulating materials commonly used inelectronics fabrication should be useful in these fabricationtechniques.

Other details concerning the present invention may be found in thepresent inventors copending U.S. patent application entitleMAGNETORESISTIVE LINEAR DISPLACEMENT SENSOR, ANGULAR DISPLACEMENTSENSOR, AND VARIABLE RESISTOR, filed on even date herewith, Ser. No08/130,480, based upon Navy Case No. 75,407. The entirety of thatcopending application is incorporated herein by reference, for allpurposes.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A method of fabricating a non-volatileferromagnetic random access memory element, comprising the stepsof:depositing a multilayered structure including:a first ferromagneticlayer and a second ferromagnetic layer, said first ferromagnetic layerhaving a magnetic moment that is co-planar with said first ferromagneticlayer, said second ferromagnetic layer having a magnetic moment and twopreferred axes of magnetic orientation for said magnetic moment of saidsecond ferromagnetic layer, wherein one of said preferred axes ofmagnetic orientation is closer to a parallel alignment with saidmagnetic moment of said first ferromagnetic layer than is said other ofsaid preferred axes of magnetic orientation; a non-magnetic metalliclayer sandwiched between said first and second ferromagnetic layers; afirst end conducting layer defining an upper end surface of saidmultilayered structure; and a second end conducting layer defining alower end surface of said multilayered structure, onto an upper surfaceof an insulating substrate; depositing a first photoresist layer over anupper surface of said multilayered structure; photolithographicallydefining a first set of lines in said first photoresist layer;selectively removing portions of said first photoresist layer defined bysaid lines to expose corresponding portions of said upper surface ofsaid insulating substrate; removing said first photoresist layer toprovide an assembly having an upper surface of lines of saidmultilayered structure upon said upper surface of said insulatingsubstrate; depositing an insulating layer to planarize said uppersurface of said assembly; depositing a second photoresist layer uponsaid planarized surface; photolithographically defining lines in saidsecond photoresist layer, said lines in said second photoresist layertraversing said lines of said multilayered structure; depositing a layerof conductive metal over said second photoresist layer to fill saidlines in said second photoresist layer; removing said second photoresistlayer.
 2. The method of claim 1, wherein said portions of firstphotoresist layer are selectively removed by ion-milling.
 3. The methodof claim 1, wherein said first and second ferromagnetic layers compriseFe, Co or Ni.
 4. The method of claim 1, wherein one of said first andsecond ferromagnetic layers is a hard ferromagnetic metal and the otherof said first and second ferromagnetic layers is a soft ferromagneticmetal.
 5. The method of claim 4, wherein said hard magnetic metal hascoercive field of at least 100 Oe and said soft magnetic metal has acoercive field of less than 100 Oe.